Two great physics by Michael Schick. The first is 20 minutes of physics. The second covers basic physics, knobology and artefacts, all in 16 minutes
If you prefer having something to read rather than watching videos, this is 14-page publication provides an excellent overview of basic ultrasound physics as well some more advanced concepts.
If you're starting from scratch it might be best to use something more basic initially and read this to advance you knowledge.
B-mode US: Basic Concepts and New Technology
The first 20 minutes of this video are dedicated to a basic introduction to ultrasound physics. It also includes some detail of probe manipulation
An excellent 35 minute summary of ultrasound physics from Mike and Matt